I really don't think that is possible with the current releases, if so someone have their own build i think.

What I do is simply run another instance of Bitcrack in another CMD window, you need to run cudaInfo.exe before to know what ID each card have.
Running more instances requires some tweaking to your parameters, as the Bitcrack application sometimes (under initialising) can get an error, i think this is a memory issue, so you need to scale down parameters for each card a bit, and test that it runs properly